Jephtha is an allusion to the book of Judges in the Old Testament. Jephtha was a military leader, and in one particular battle, he vowed that if he were successful, he would sacrifice to god the first person who greeted him at his return home. The person who greeted him just happened to be his daughter, but the distraught Jephtha kept his word. When Hamlet calls Polonius Jephtha, he is implying that Polonius is using his daughter to advance his own career. And, indeed, Polonius is using his daughter Ophelia. He shares the love letters she received from Hamlet with Claudius, and plots to use Ophelia to uncover the nature of Hamlet's madness--all to curry favor with the king.
Hamlet also calls Polonius a "fishmonger." Although the second meaning of this term is disputed among critics, many believe that "fishmonger" was another word for pimp. Again, Hamlet is implying that Polonius is using his daughter selfishly.